1. Field of the Invention
This invention relates generally to the control of a vehicle stopped on a hill, particularly to holding the vehicle against unintended rolling.
2. Description of the Prior Art
In micro hybrid vehicles where engine is shut down when the vehicle is stopped it is desirable to prevent the vehicle from rolling backwards when vehicle is on an uphill grade. This is particularly critical when the brake pedal is released but the engine is in the process of starting up and has not developed full torque.
At the same time when the vehicle is stopped on a downhill slope it is not necessary to inhibit vehicle motion since it is common driver expectation to see vehicle rolling on the downgrade road when brakes are released.
In some existing art this is accomplished through a hill hold system in which the wheel brakes are applied. These systems require grade detection, which can be challenging due to various electronic noise factors such as temperature and time drift of the grade sensor signal, and various failure modes when the sensor information is not available to the brake system.
Some brake hill hold systems also require an electric pump to create either hydraulic pressure or vacuum, which maintain excessive brake pressure once the brake pedal is released. This pump depletes the vehicle battery and thus reduces potential fuel economy benefit.